Initiator primer



p l 7, 1959 H. J. LUTZ EI'AL INITIATOR PRIMER Filed Oct. 5. 1954 ROBERT F. DORAN HENRY a. LUTZ v w cg g u x 5 v 6% M1.- I... :5... i xv ww 5%:5iii g INVENTORS ATTORNEY? INITIATOR PRIMER Henry J. Lutz, Bayshore, and Robert F. Doran, Floral Park, N.Y., assignors to Sylvania Electric Products Inc., a corporation of Massachusetts Application October 5,1954, Serial No. 460,439

1 Claim. (Cl. 102-28) This invention relates to primers for initiating explosive discharges.

In many industries and in warfare it is desired to fire a bulk of an explosive mass by initial firing of a much smaller charge of explosive. Where the explosive material is expensive or the unit to be exploded is expensive to produce or where it is desired to ensure the absence of duds, great care is involved in the construction of the initial primer.

In a certain type of primer a measured charge of primary explosive is ignited by an electrically heated very fine wire. This wire in an intended application of the primer is a tungsten wire of the order of .0001" in diameter, and has a length of .l25":.062 with a resistance of close to 275 ohms to the inch. The eliective length of the wire in the finished primer, i.e, the length of wire effective to fire the charge, would be of the order of .015. To insure uniform results, the electrical potential applied to the wire, the diameter of the wire, and its length are precisely set forth in specifications. In the past it has been diflicult to maintain these specifications principally because the length of the wire is small and small variations in length create large percentage differences in heat. Consequently erratic results with respect to the explosion of the primary charge were obtained. An insufficient amount of heat in the wire would create a dud and an excess amount of heat developed might cause undesired premature explosion of both the primary charge and the main explosive mass. In the case of a missile embodying the explosive, this might cause failure to strike the target.

It is an object of this invention to provide a method for forming primers which shall insure uniform results as to firing of an initial explosive charge.

It is a further object of the invention to provide a primer in which the effective portion of the heating element is maintained at a definite length.

It is a still further object of the invention to provide a construction of heating element and associated parts such that there is little likelihood of breakage of the element in assembling the primer, particularly when the explosive charge is loaded into the primer.

These and other objects will be apparent after reading the following specification in conjunction with the accompanying drawing in which- Fig. l is a vertical cross section through an assembled initiator primer.

Fig. 2 is a vertical sectional view through the inside shell assembly of the initiator prior to assembly within the outer shell.

Fig. 3 is a perspective view of the filament carrier in an initial stage of manufacture.

Fig. 4 is a similar view in an intermediate stage of manufacture, the filament being added.

Fig. 5 is a plan view of the filament and carrier mounted in place on the inner shell assembly with the carrier still in an intermediate stage of manufacture and United States Patent 0 Fig. 6 is a similar view with the carrier processed to its final form.

Referring to the figures in greater detail, it will be seen that the initiator primer is comprised of four main portions. These are an outer metallic shell 10, an inner shell assembly 12, a charge retaining insulating sleeve 14 and an explosive charge 15.

The outer shell 10 comprises a cylindrical tube closed at the upper end and having the lower edge turned in to grip inner parts, as later described.

The inner shell assembly comprises a metallic cylinder 16 with a flared cylindrical skirt portion 18. Within the upper end portion of the cylinder, opposite the flared portion, is an insulating plug 20, as for example of polystyrene, said plug having a central frusto conical protuberanee 22, the upper face 24 of the truncated cone being perfectly flat. Traversing the plug and secured thereto is a pair of conductors 26, to the upper ends of which are secured a pair of plate members 30, 30 carrying a heater wire 32. The plate members are secured to the conductors by means of flanged portions soldered, spot welded or cold swaged to the conductors. The lower ends of the conductors may be left free for soldering to other portions of the device with which the primer is to be used or may be finally associated with plug-in pins, as will be described later. The flanged plates and heater wire are formed as follows: Initially a flat rectangular metallic plate 28, see Fig. 3 has its corners 29, 29, shown in dotted lines, cut away leaving end sections 30. The free ends of these end sections are then bent down on the dotted lines indicated at 31 to form downwardly depending flanges. A slot 34 having parallel edges is cut across the width of the plate and substantially midway of its length, the width of said slot being precisely selected in accordance with the effective length of wire desired. This slot although it may be cut prior to bending of the plate, is preferably cut after the bending operation described to prevent distortion of the slot as might occur if the bending were performed subsequent to slot formation. Next, the wire 32 is laid perpendicularly across the slot and cold swaged into good electrical contact with the end portions 30 of the plate. Obviously, other definite angles for laying the wire across the slot may be selected to give other definite effective lengths of wire. It will be noted that during all of the operations so far described the tab 36 holds the parts 30, 3D in fixed relationship to one another to maintain the solt 34 unaltered in width. After the cold swaging operation the formed assembly is mounted on the protuberance 22 with the under face of the plate resting on the protuberanee. The flanges on the plate portions 30 are then soldered or otherwise electrically and mechanically secured to the conductors 26. At this time, it will be noted, the bridge portion of the heater wire 32 is spaced very slightly above the upper face 4 of the protuberance. In this stage of manufacture, the heater wire 32 is short eircuited by the tab 36. Now, however, the tab is cut off on a line crossing the slot 34, as on a line following the closed end of the slot, to remove the short circuit around the wire. The resultant structure is shown in Fig. 6. By the method and means hereby described the effective portion of the heater wire 32 has been maintained straight and at a fixed length, since the slot 34 has been maintained unaltered in width throughout the process of forming the plate and mounting it on the conductors 26.

After the plate assembly has been put in place, as de scribed, the insulating sleeve 14, which may be of polystyrene, is slipped over the shell assembly 12. This sleeve 14 has a cross partition wall 38 about midway of its length, the wall being perforated at 40 to allow explosive powder 15 to sift into the area below and above the heater wire. The bottom of the wall abuts the swagejd-in wire .32 andendpieces 30. It-shouldbe understood that the swaging operation has left the upper surface of the wire in substantially coplanar relationship to the upper surfaces of the end pieces. A measured amount of thelexplosivei15;is,packed in ,the uppercup like portion of the sleeve 14. This when exploded by the heat fromthe wire 32 willrupture the. outer shell 10 and fire the main charge. Theouter shell 10,is slipped over the polystyrene sleeve 14 and. suitably fastened to the skirt portion 18 of the cylinder 16. A convenient way to do this is by rolling or peening over the lower edge of the outer shell against the lower edge of the skirt portion. If desired, .the lower ends of the conductors 26 may be soldered to plug-in pins 42 suitably mounted in an insulating plug 44 retained in, the ,skirt. ,portion, .as for example by reasonof the;r0lling .in of the. lower edges of the outer and ,inner :shells and ;the flare immediately above the. skirt portion .18..

it should be understood that various modifications in the above described "method. andtstructure may ,be -.made without departing ,from the principles of this invention as defined by the appended claim'and it is intendedthat all such modifications as fall within the terms of the claim shall be covered by theclaim.

Having thus described our invention, what we claim as new is:

In an initiator primer, :a heater wire assembly comprising an insulating body and a protuberance ext-ending thereabove, said protuberance having a planar top, a pair of conductors supported by and extending above the insulating body and alongside of the protuberance, a pair of plate members extending parallel to said planar top with the lower surfaces thereof resting on said protuberance but spaced apart on said protuberance to leave a gap between the members, said members having flanges extending along the conductors and electrically and mechanically joined thereto, and a heater wire on the upper surfaces of said plate members-and spanning-the gap therebetween to leave a space between the protuberance and the wire for retention of explosive.

References Cited in the file of this patent UNITED STATES PATENTS 2,226,988 Woodworth Dec. 31, 1940 2,230,080 Johnson et a1. Jan. 28, 19.41 2,269,195 "Finlayson Jan. 6, 1942 2,279,451 English et al Apr. '14, 1942 2,481,696 Seavey Sept. I3, 1949 2,596,325 Cerny May'l3, 1952 2,762,302 MacLeod Sept. 11, 1956 FOREIGN PATENTS 284,085 Switzerland Nov. 1, 19552 

